Norma L. Fowler

Effects of White-tailed Deer (Odocoileus virginianus) on Plants, Plant Populations and Communities: A Review

Abstract Large effects of white-tailed deer (Odocoileus virginianus) upon individual plants, plant populations and communities have been documented in a number of studies. However, well-supported experimental measures of the magnitude and geographical extent of these effects are still surprisingly scarce. Deer-caused changes in stem morphology and reductions in plant growth rates are well-documented in some parts of the North America. Furthermore, deer have been shown to affect the composition of several plant communities in the north-central and northeastern United States. There are some documented cases of deer-caused reductions in plant survival; most of these are tree seedlings and saplings. However, many studies have detected no effects on plant survival or fecundity, or have found that negative effects occur only in a fraction of years, seasons, sites or deer densities. Little is known about population-level or ecosystem-level impacts. Many regions and plant communities with large deer populations have not been studied. Whereas deer density is clearly important in determining spatial and temporal variation in the presence and magnitude of deer effects, other factors that may modify the effects of deer density are poorly understood.

WHAT IS A SAFE SITE?: NEIGHBOR, LITTER, GERMINATION DATE, AND PATCH EFFECTS'

To investigate some of the factors that determine the fate of a seedling, censuses of seedlings of two native grasses, Aristida longiseta and Bouteloua rigidiseta, were made at frequent intervals in a Texas grassland for 2 yr. Seedlings of both species with neighboring seedlings or juvenile plants of either species within 2 cm had higher rates of survival and growth than those without such neighbors, implying that the effects of aggregation in favorable microsites outweighed the effects of competition among these plants. Neighboring adults of these species, and neighboring juveniles of other species, had positive effects in some instances, negative in others. Litter, even in amounts too small to cover the surface, reduced the survival rates of seedlings of both species, while surface rocks may have increased them. Overall, seedlings that germinated earlier in the autumn were more successful. The rank order of quadrat favorableness for seedlings varied in a complex fashion between species, between years, among census intervals, and among germination cohorts. This variation in patch favorableness must prevent these populations from reaching a stable spatial distribution and from reaching a stable age distribution in any given patch.

Microsite requirements for germination and establishment of three grass species

The safe sites, i. e., microsite requirements for germination and establishment, were partially characterized for three of the dominant grass species of the Edwards Plateau of central Texas, Aristida longiseta, Bouteloua rigidiseta and Stipa leucotricha. Seeds were germinated and grown in field-collected soil in a greenhouse under different watering regimes and in soil-filled benches with various soil surfaces and with adult plants. Both watering regime and microsite type strongly affected germination and establishment. Litter and rocks, but not the proximity of an adult plant, increased germination, survival and growth. A safe site for these species appears to be a microsite that prevents desiccation. Differences among the species in the relative favorableness of different types of microsites were slight. Species differed more in the timing of germination. Since the timing of rainfall and of temperature fluctuations varies considerably between years in central Texas, different cohorts of seedlings, and hence different species, may be successful in different years.

Ecological Constraints on the Establishment of a Novel Polyploid in Competition with Its Diploid Progenitor

Competition between a newly formed polyploid and one of its diploid parental species has been modeled. Models were based on the Lotka-Volterra competition model, to which were added terms representing the positive frequency-dependent effect (minority disadvantage) as a result of lowered fertility caused by the receipt of inappropriate pollen from the other cytotype. A new polyploid may persist by replacing its diploid parent as a result of the combination of a very small diploid population, the existence of an unstable equilibrium between the two cytotypes, and chance events (probably the most common route); by coexisting with it as the result of niche separation; or by outcompeting and replacing it (probably very rare). However, the conditions for all of these possibilities are quite restrictive. The model and results are applicable in some degree to the establishment of other novel cytotypes, and also to pairs of species that compete for resources and also experience minority disadvantage (perhaps because of competition for pollinators) with respect to each other.

Competition and coexistence in a North Carolina grassland. II. The effects of the experimental removal of species.

(1) Species were removed singly (by hand) or in groups (by herbicide or soil sterilization) in a mown field. The vegetation of treated and control quadrats was assessed before and after the removals by measuring the cover of each species. (2) In all cases total cover returned to its original value within a year, although the species composition remained different. (3) The effects of the removals of single species upon the abundance of the other species were small, accounting for an average of only 7% of the variance. Fourteen of the seventy-two pairwise effects tested were significant. Interactions between the different pairs of species did not differ greatly in magnitude. The results of this experiment do not support the hypothesis that this community is divided into well-defined groups of competing species (guilds). A previous study provided evidence for the division of the community into temporal guilds. But within each temporal guild the species are characterized by diffuse competition. (4) Interactions between pairs of species were in general non-reciprocal. (5) Some interactions were found among three or more species that were not predictable from interactions of pairs of the constituent species. Two species with vigorous vegetative spread pre-empted space from competitors, and there were several significant negative responses to species removals. (6) No divisions of the community along taxonomic or morphological lines were found. However, the temporal guilds correspond to the distribution of the C3 and C4 photosynthetic pathway in the grass species. (7) The spatial patterning of the community showed no relationship to the responses to removals.

PLASTICITY OF YIELD COMPONENTS IN RESPONSE TO STRESS IN SESBANIA MACROCARPA AND SESBANIA VESICARIA (LEGUMINOSAE)

The effects of limited moisture, limited nutrients, and defoliation on the subsequent allocation to yield components were measured in Sesbania macrocarpa and S. vesicaria in order to test the hypotheses that all yield components might be involved in the regulation of reproduction and that patterns of plasticity in reproduction may be species-specific. In S. vesicaria, the number of fruits per flower was the only yield component consistently reduced below the control. In S. macrocarpa, the responses to mild stress were reductions in the number of seeds per ovule and in seed weight, components of yield probably determined later than the number of fruits per flower. Severe stresses resulted in a change in flower number. Both the pattern and the amount of plasticity in yield components differed significantly between the species. Differences in responses to the same stresses may represent different adaptations resulting from variation in the importance of each component of yield for the successful reproduction of the two species in slightly different habitats. Within species, half-sib families differed in the amount, but not in the kind, of plastic response. Each species appears to be genetically invariant for plastic response in the same yield component, fruit survivorship, even though the overall pattern of plastic response differed between the species. Perhaps developmentally early yield components are genetically fixed to be either variable or stable, and responses in developmentally later yield components are determined by the levels of change in the earlier yield components.

Joint effects of competitors and herbivores on growth and reproduction in Aristolochia reticulata

The separate and joint effects of herbivory and interspecific competition on an herbaceous plant were measured to determine to what extent, if any, herbivory and competition interact in their effects, and to test models of their joint effects. Plants of Texas Dutchmans pipevine, Aristolochia reticulata, were grown in a greenhouse, alone and in competition with either or both little bluestem grass, Schizachyrium scoparium, and southern dewberry, Rubus trivialis. Herbivory on A. reticulata by the pipevine swallowtail butterfly, Battus philenor, and on S. scoparium by cattle was simulated by clipping. Competition and clipping individually had the expected negative effects on A. reticulata growth and reproduction. The joint effect of competition and clipping on A. reticulata was best described as additive. The joint effect of the two competing species on A. reticulata in three—species mixtures was well predicted by a simple, but nonlinear, model that assumed that all three species competed for the same limit...

Small‐Scale Variability in the Demography of Transplants of Two Herbaceous Species

Individuals of Plantago lanceolata and Salvia lyrata, both herbaceous perennial species, were transplanted into plots of otherwise unmanipulted vegetation. Individuals came from one of 12 half—sibships (common mother). Individuals were marked and leaf and inflorescence numbers counted at intervals for 14 mo. Consistent differences among plots, in survivorship, size, and reproduction, were found in both species, although the plots were only metres apart. Causes of this spatial variation are discussed. Evidence for genetic and genetic x environmental variation in Plantago lanceolata is suggestive but not conclusive. See full-text article at JSTOR

PLASTICITY IN YIELD COMPONENTS IN RESPONSE TO FRUIT PREDATION AND DATE OF FRUIT INITIATION IN THREE SPECIES OF SESBANIA (LEGUMINOSAE)

SUMMARY (1) Changes in the components of yield in response to artificial predation and to date of fruit initiation were measured in three species of Sesbania. (2) The removal of fruits, which mimicked damage by chewing insects, resulted in significant changes in seed weight but not in seeds produced per fruit. Injection of alcohol into developing seeds, which mimicked damage by hemipterans, resulted in selective abortion of more heavily damaged fruits and of younger fruits. (3) The nature of the response to predation depended on its timing because responses become less flexible as development proceeds. (4) Differential responses to predation among the species appeared to be related to the investment in seeds versus pods, since S. vesicaria, which had the highest quotient of seed weight to fruit weight, had the highest rate of fruit abortion. (5) Fruits initiated on later dates were less likely to mature in all three species. Late fruits had an unchanged number of ovules but reduced seed size in S. macrocarpa (annual), reduced ovule number but unchanged seed size in S. vesicaria (annual), and unchanged ovule number and seed size in S. drummondii (perennial). (6) The differences among species in response to date of fruit initiation suggest that each annual species holds most constant the yield component most closely related to fitness, while the perennial is less responsive to stress because its stored reserves provide a buffer.

Wide Ecological Amplitude of a Diversity-Reducing Invasive Grass

The goals of this study were to identify habitats occupied by the Eurasian grass Bothriochloa ischaemum (King Ranch bluestem) on the eastern Edwards Plateau of central Texas, USA, and to measure the effects of this invasive species on plant diversity. A set of descriptive field studies were conducted to determine the relationships between habitat characteristics (woody cover, slope, presence of roads and trails, history of prescribed burning, and presence of grazing) and the presence and abundance of this species. Species richness and diversity of perennial herbaceous species were compared between plots in which B. ischaemum was absent and plots that it dominated.The only habitat in which B. ischaemum was never found was under the canopies of woody plants. B. ischaemum grew in plots of all inclinations (flat sites to steep hillsides), with little evidence of habitat preference. B. ischaemum was more often, although not exclusively, found in plots near roads, probably because roads facilitate seed dispersal. There was no significant relationship between either grazing or fire history and the distribution of this species. B. ischaemum-dominated plots had lower species diversity and species richness than plots in which no B. ischaemum was found. The results of this study confirm that B. ischaemum is very common on the eastern Edwards Plateau and suggest that it will continue to spread throughout this region and to reduce native herbaceous plant diversity there.